1. Field of the Invention
The invention generally relates to optical transport systems and more particularly relates to an optical time domain reflectometer integrated with an optical supervisory channel.
2. Related art
Spurred by the growth in information technology and the need to transmit large amounts of data, fiber optics has become the backbone-networking technology of choice in the last decade. In the arena of fiber optics, the idea of using multiple wavelengths to transmit information in a single fiber, called Dense Wave Division Multiplexing (DWDM), combined with the amplification in the optical domain has reduced the cost of building-out large optical networks. This has been achieved by reducing the need to install electrical regenerator equipment every few kilometers; optical amplifiers increase the power of all channels passing through them. This, together with the increase in the spacing between optical amplification equipment to 60-80 kilometers in the newer networks, resulted in important cost savings.
However, large-scale networks with such advanced technology components have made testing and maintenance of these networks a complex, yet inevitable procedure. The various components in the networks such as multiplexing/de-multiplexing equipment, optical amplifiers, and the fiber used to connect these components have to be tested to ensure they are operating at the peak of their capabilities.
Testing of the optical network components occurs when the networks are being installed, maintained and during trouble-shooting problems.
One of the recent advancements in the area of testing fiber characteristics is the use of Optical Time Domain Reflectometers (OTDR). The OTDRs measure the loss of optical signal strength in a section and the total loss encountered in an end-to-end network by tracking the attenuation in the optical signal. The OTDR operates by launching a short pulse of light of a predetermined wavelength (frequency) into the fiber, and measuring the reflected signal as a function of time.
Reflections are mainly due to the phenomena known as Rayleigh scattering and Fresnel reflections, which cause that a part of the transmitted optical signal to be reflected back via the optical fiber. The Rayleigh effect relates to power attenuation along the fiber due to back and forth scattering, fiber imperfection, and absorption of light due to impurities in the fiber. The Fresnel effect relates to the interference of two light spots generated by the same source, the reflected spot being delayed relative to the original spot.
Due to back and forth scattering in the transmission medium, delays are introduced in the reflected waves. Such delays are induced by the fiber characteristics, manufacturing nonlinearities, reflections at the optical connectors, splices, kinks or breaks, etc. If a detector is located on the same optical path as the source, the Rayleigh scattering may be measured with accuracy, and knowing the speed of the light, the spatial location of such events can be determined.
The OTDR measurements provide an OTDR trace which helps identify the distance between irregularities, and their contribution to optical signal degradation. This instrument can detect extremely weak reflections from fiber breaks with about 50 centimeters distance resolution, and also estimate the attenuation of the optical signal relative to signal position in the fiber. The OTDR provides this information non-destructively, from one end of the fiber. Further on, the information is used for testing and monitoring the optical system components during normal system operation.
For example, based on OTDR measurements, the splice loss (reflectance) can be estimated and this information is used to detect alignment of fibers during splicing, degradation profiles and breaks or damages in the fiber. OTDRs are also used to do pre-installation testing, acceptance testing, predictive maintenance, and/or troubleshooting in optical networks.
Typically, the OTDR equipment is provided as a separate (ex-network) testing device, and operates generally when there is no traffic on the fiber under test, because the diagnostic pulses interfere with the traffic signal. When there is a need to characterize a fiber link, the head-end of the fiber is disconnected from the network device and plugged into an OTDR port. As mentioned above, such monitoring techniques are tied to an end of the optical fiber connection and also require intervention in the transmitting equipment.
An in-service OTDR device is described in U.S. Pat. No. 5,570,217 issued Oct. 29, 1996 to Fleuren. The OTDR technique described by Fleuren allows system monitoring during operation at any point where the optical fiber is accessible to optical uncoupling means, such as “clip-on” procedure.
A side-tone OTDR for in-service optical cable monitoring is described in U.S. Pat. No. 5,926,263 issued to Lynch et al. on Jul. 20, 1999. When the OTDR of Lynch et al. provides in-service measurement information, the test signals (in the form of side-tone pulses) are at a low level and sufficiently offset from the wavelength of the traffic signal to minimize interference. When the device is used in a wavelength division multiplexing (WDM) system, the wavelength offset of the test signals must be small enough to pass through the narrow bandpass filter of the OTDR receiver along with the dropped/added traffic, yet be far enough away to avoid interference. In addition, in an WDM system, the out-of-service procedure may be applied to one or more traffic wavelength assignments, while other wavelengths on a fiber pair are still carrying traffic.
Both OTDR devices described above, generate pulses that travel along the fiber in parallel with the traffic and the control signals, while the OTDR trace is available only at repeaters which include the OTDR mechanism.
There is a need to provide a network with an on-line device for measuring reflections and providing a OTDR trace anywhere in the network.
In DWDM networks, a pair of wavelengths (typically at 1610 and 1625 nm) is dedicated to a bidirectional Optical Supervisory Channel. Optical Supervisory Channel (OSC) is a technique used to transport network management data between optical amplifiers. OSC functionality is usually provided for on a separate card present at amplification nodes. OSC technologies have progressed considerably in recent years. In particular, advanced data communications technologies are now used in conjunction with optical solutions to design network overlays as described for example in applicant's patent application “QoS based supervisory network for optical transport networks” Ser. No. 60/251,136, filed on Dec. 4, 2000, and assigned to the same applicant.
The OTDR is a mainstream test device. The OSC is usually designed as a part of the network equipment vendor's offering. Thus, the OTDR and OSC are two known technologies that are currently implemented by separate devices.
There is a need to simplify network engineering by providing a unique supervisory layer which combines network control and OTDR functionality on integrated network management platforms distributed within the network, to allow acquisition and monitoring the OTDR trace from anywhere in the network.